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<div class="titlepage"><div><div><h3 class="title">
<a name="math_toolkit.float128.overloading"></a><a class="link" href="overloading.html" title="Overloading template functions with float128_t">Overloading template
      functions with float128_t</a>
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<p>
        An artifact of providing C++ standard library support for quadmath may mandate
        the inclusion of <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
        <span class="bold"><strong>before</strong></span> the inclusion of other headers.
      </p>
<p>
        Consider a function that calls <code class="computeroutput"><span class="identifier">fabs</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code>
        and has previously injected <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">fabs</span><span class="special">()</span></code> into local scope via a <code class="computeroutput"><span class="keyword">using</span></code>
        directive:
      </p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">bool</span> <span class="identifier">unsigned_compare</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">T</span> <span class="identifier">b</span><span class="special">)</span>
<span class="special">{</span>
   <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">fabs</span><span class="special">;</span>
   <span class="keyword">return</span> <span class="identifier">fabs</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">fabs</span><span class="special">(</span><span class="identifier">b</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
        In this function, the correct overload of <code class="computeroutput"><span class="identifier">fabs</span></code>
        may be found via <a href="http://en.wikipedia.org/wiki/Argument-dependent_name_lookup" target="_top">argument-dependent-lookup
        (ADL)</a> or by calling one of the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">fabs</span></code>
        overloads. There is a key difference between them however: an overload in
        the same namespace as T and found via ADL need <span class="emphasis"><em><span class="bold"><strong>not
        be defined at the time the function is declared</strong></span></em></span>. However,
        all the types declared in <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
        are fundamental types, so for these types we are relying on finding an overload
        declared in namespace <code class="computeroutput"><span class="identifier">std</span></code>.
        In that case however, <span class="emphasis"><em><span class="bold"><strong>all such overloads
        must be declared prior to the definition of function <code class="computeroutput"><span class="identifier">unsigned_compare</span></code>
        otherwise they are not considered</strong></span></em></span>.
      </p>
<p>
        In the event that <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
        has been included <span class="bold"><strong>after</strong></span> the definition of
        the above function, the correct overload of <code class="computeroutput"><span class="identifier">fabs</span></code>,
        while present, is simply not considered as part of the overload set. So the
        compiler tries to downcast the <code class="computeroutput"><span class="identifier">float128_t</span></code>
        argument first to <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>,
        then to <code class="computeroutput"><span class="keyword">double</span></code>, then to <code class="computeroutput"><span class="keyword">float</span></code>; the compilation fails because the result
        is ambiguous. However the compiler error message will appear cruelly inscrutable,
        at an apparently irrelevant line number and making no mention of <code class="computeroutput"><span class="identifier">float128</span></code>: the word <span class="emphasis"><em>ambiguous</em></span>
        is the clue to what is wrong.
      </p>
<p>
        Provided you <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
        <span class="bold"><strong>before</strong></span> the inclusion of the any header containing
        generic floating point code (such as other Boost.Math headers, then the compiler
        will know about and use the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">fabs</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">float128_t</span><span class="special">)</span></code>
        that we provide in <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>.
      </p>
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